This disclosure relates to vertical takeoff and landing (VTOL) rotorcraft, in general, and in particular, to a high speed, low drag, low maintenance, stiff in-plane, gimbaled rotor head for a helicopter that enables three or more rotor blades to be used per rotor, while enabling a more compact tandem rotor helicopter by allowing greater blade intermesh through the elimination of the lead-lag motions and dampers associated with the fully articulated rotor heads of the prior art.
A fully articulated type of rotor head 100 of the prior art, such as used on the Boeing CH-47 “Chinook” tandem rotor helicopter, is illustrated in FIG. 9. As illustrated therein, the fully articulated rotor head comprises a rotor shaft 102, a hub 104 disposed at the end of the shaft and three blades 106 radiating outward from it. Each of the rotor blades is pinioned to the hub by a pair of hinges, viz., a “flap” hinge 108 and a “lag” hinge 110 that respectively enable the associated blade to pivot both up and down, and fore and aft, relative to the hub. Additionally, the blades and lag hinges are rotatably coupled to respective flap hinges by respective pitch shafts 112 rotatably disposed within respective pitch shaft housings 113 for conjoint rotation with the respective blades about the respective long axes thereof. Thus, in addition to being capable of up and down and pitching movements, the blades are also capable of lead-lag movement in the plane of rotation of the blades, i.e., they are “flexible in-plane.”
A disadvantage of flexible in-plane rotor head designs when used in tandem rotor aircraft is that it is difficult to get the respective blades of the two rotors to intermesh with each other when rotating in the same plane due to the range of angular displacement that each blade may undergo within its respective plane of rotation. As a result, the two rotors must be spaced apart from each other, either horizontally or vertically, such that the respective blades do not overlap, or their respective planes of rotation are not coplanar.
In order to overcome this drawback, efforts have been made to develop “stiff in-plane” rotor hubs, i.e., hubs with blades that are incapable of pivotal movement in the plane of rotation of the blades. The existing solutions for stiff in-plane hubs are the so-called two-bladed “teeter” hubs, such as used on many light rotorcraft, and three-bladed gimbaled hubs, such as are used on the Bell-Boeing V-22 “Osprey” hybrid tilt-rotorcraft.
Studies have shown that rotors having greater solidity are required for next-generation, high speed, heavy lift, tandem rotor helicopter designs. This greater rotor solidity is most efficiently delivered with a large number of blades (as many as 6 blades per head). Teeter rotor heads inherently can employ only two blades per hub, and are therefore unsuitable for high speed, heavy lift helicopter configurations. The use of stiff in-plane hubs enables a larger number of blades (more than three) of the two rotors to intermesh tightly when rotating in the same plane so as to keep the configuration compact and performance high, while at the same time avoiding the limitations of the flexible in-plane hubs of the prior art.
Accordingly, there is a need in the rotorcraft field for a high speed, low drag, low maintenance, stiff in-plane, gimbaled rotor head for a high-speed, heavy lift helicopter that achieves a more compact tandem rotor blade intermesh by eliminating the lead-lag motions and dampers used in the fully articulated rotor heads of the prior art, and that also enables more than three rotor blades to be used per rotor.